Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on reaction

  1. Jan 30, 2006 #1
    my teacher on fire estinguising subject explained me that with the same pump if you reduce the size of the nozle you increase reaction on the fire stinguisher

    so does reaction depend on cinetic energy sent away?

    but if by halving the nozle you get double force of reaction with the same pump, cant you half the nozle infinitally with ideal materials and fluid, and double the force of reaction infinitally

    then what would avoid you from using this force of reaction to feed the pump since this force in theory can be infinite by making the nozle infinitesimal (this is what i understand from my teachers explanation)
  2. jcsd
  3. Jan 30, 2006 #2
    I like your thinking.. The theory of halving nozzle size and doubling pressure or force of reaction works only for sizes bigger than some limit say 2cm in diameter.
    Apart from pressure from inside, the liquid/gas flowing in the air will be expering friction-type forces on the walls of the nozzle

    When we reduce the diameter of nozzle, these frictional forces will be huge and they will overcome the pressure. and will stop the liquid coming out.

    You can try this will a syringe/injection. Without the needle, water comes fast, with needle it takes some time because the needle is so thin that it have more friction-force against liquid coming out. (these forces are also called vanderwall forces and property is called viscosity)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook